Security forces have so far been unable to retake Tikrit but claim momentum has begun to shift after soldiers, backed by air strikes from a US-led coalition, recently seized the nearby oil refinery town of Beiji.

Controlling Tikrit would probably be necessary for any attempt to attack Mosul because of its strategic location for the flow of arms and military enforcements.

Al-Iraqiya state television today reported that Iraqi government soldiers and Sunni and Shia militias allied against Isis were striking the city from different directions, backed by artillery and air strikes by Iraqi fighter jets.

Militants were reportedly dislodged from some areas outside the city but no further details were given.

Hours before the operation began, Shia Prime Minister Haider al-Abadi, called on Sunni tribal fighters to abandon the extremist group, offering what he described as “the last chance” to surrender and promising them a pardon.An Isis propaganda video purporting to show fighters near the Iraqi city of Tikrit

“I call upon those who have been misled or committed a mistake to lay down arms and join their people and security forces in order to liberate their cities,” he said on Sunday during a news conference in Samarra, 60 miles north of Baghdad.

“The city will soon return to its people.”

His comments appeared to target former members of Iraq’s outlawed Baath party, loyalists to Saddam who joined Isis during offensive, as well as other Sunnis who were dissatisfied with Baghdad’s Shia-led government.

Saddam, who ruled Iraq for more than 23 years until he was ousted in 2003 during the US-led invasion, was executed in 2006. Saddam Hussein drew large support from Tikrit

He drew his support from Iraq’s minority Sunni population and since his capture and death, the government has been led by Shias, continuing the bitter divide between the two Muslim traditions.

The alliance of both sides in the Tikrit operation is seen as an important development in the battle against Isis but the presence of Shia forces in Sunni areas could prompt a backlash in the future.

Isis ascribes to a violent Salafist interpretation of Sunni Islam and has been known to execute Shias, reportedly labelling them apostates and using questions about prayer methods to single them out.

Tikrit has been a frequent battleground in a series of invasions over hundreds of years.

They controversially made Saddam’s palace complex their headquarters but handed it over to the local government in 2005.A picture posted on a militant website reportedly shows jihadists from Isis taking aim at captured Iraqi soldiers wearing plain clothes, after taking over a base in Tikrit

Isis has controlled the town since last summer and made its presence felt in September with the destruction of the 7th Century Assyrian Green Church and Forty Shrine, the oldest Islamic shrine in Iraq.